Just download and install Firefox 3.1b3 and it will install in it's own folder (c:\Program Files\Mozilla Firefox 3.1b3\) and keeps 3.0.8 in it's original (C:\Program Files\Mozilla Firefox\). Then test 3.1b3, if you find it stable enough and don't want to use 3.0.8 anymore then uninstall it. When going to uninstall 3.0.8
DO NOT check mark Remove my personal data and customizations....Or you will loose your add-ons and such. So don't check mark that when uninstalling 3.0.8.
As for updates. It updates the same as non-beta Firefox does......Automatically. So it will download and install 3.1b4 when it comes out. Or hopefully 3.1 final is next.
It's beta so it *can* crash and maybe do *odd* things. However so could 3.0.8 with a certain combo of add ons. So try it and if it acts up and you can't stand it, then stick with 3.0.8, I haven't noticed any major issues.